The molecular basis of heredity
gene structure and its function.Characteristics of the human genome.DNA - the molecule of heredity.Chemical and structural features
Already from the biology curriculum is known that the gene - a segment of the DNA molecule.Only this macromolecule fairly broad spectrum of macromolecules present in every cell of every living organism, can replicate, and therefore to transfer in cells or organisms generation information contained therein.The ability of DNA to reproduce due to the peculiarities of its chemical structure.The DNA molecule is built from three components: a sugar represented deoxyribose, phosphate groups, and 4 types of nitrogenous bases - cytosine (C), thymine (T), which is also called purines, adenine (A) and guanine (G).This - pyrimidines.
In 1953, Watson and Crick published their historical article about the physical structure of DNA.According to the model of Watson and Crick, the DNA molecule is a double helix.Each spiral twists around other spiral along a common axis.Chains of this sp
The great thing about the DNA molecule that every nitrogenous base can connect only with other well-defined and complementary (suitable only for him) his base, namely, adenine with thymine and guanine with cytosine.
This property nucleotides complementary pair provides the basis for accurate reproduction of the nucleotide sequence of each DNA strand.The nucleotide DNA strand polar.The polarity is determined by how interconnected sugar (deoxyribose).Phosphate group attached to the C5 (five-carbon) of the sugar, is connected to a hydroxyl group at position C (3-carbon) sugar next by a phosphodiester bond.As a result, the terminal nucleotide at one end of the chain has a free 5, and the other - the free 3 group.The sequence of nucleotide bases is usually written in the direction from 5 'end to 3.Two DNA strands are antiparallel to each other, as are in opposite directions and the end 5 of one chain corresponds to 3 'end of another chain and vice versa.
Model of DNA Watson and Crick explained by the time a well-known British biochemist Chargaff rule, according to which any molecule of DNA amounts of purines strictly corresponds to the number of pyrimidines.
In the double helix of DNA puriny- (adenine, guanine) is always connected to the pyrimidine (thymine and cytosine).Between cytosine and guanine forms three hydrogen bonds between thymine and adenine, and - two base so otherwise simply can not connect.
Elementary unit of DNA is a nucleotide which includes a deoxyribose, a phosphate group and one basic nitrogen.
Since DNA is the molecule of heredity, for the implementation of this quality it must accurately replicate itself and thus save all information in the original molecule of DNA information in the form of a specific nucleotide sequence.This is achieved by a special process prior to the division of any body cells, called DNA replication.The essence of this process lies in the fact that a special enzyme breaks the weak hydrogen bonds that connect together the two strands of nucleotides.As a result of the DNA strand and are separated from each chain of "stick" the free nitrogenous bases (the appearance of the so-called replication fork).Particular DNA polymerase enzyme begins to move along the DNA chain from the free 5- to 3-end of the molecule, helping to join the free nucleotides continuously synthesized in the cell to the 3 end of the newly synthesized DNA strand.
As a result, replication formed two new, exactly the same DNA molecule, identical as the original DNA molecule prior to its reduplication.
can say a few simplifying the phenomenon exact doubling of the DNA molecule, which is based on the complementarity of the bases of this molecule is the molecular basis of heredity.
rate of DNA replication in humans is relatively low, and to provide DNA replication any human chromosome, would take weeks, if replication is started from the same point.In fact, any molecule of DNA chromosomes, and each chromosome person has only one DNA molecule, there are many places of initiation of replication (replicon).From each replicon replication in both directions as long as adjacent replicons do not merge.Therefore, the DNA of each chromosome replication proceeds relatively fast.
term "genetic code»
For a molecule of heredity, which is DNA not only that she is able to reproduce itself - is only part of the inheritance.DNA must somehow encode all the hallmarks of an organism.Most of the features of any organism a unicellular or multicellular determined proteins: enzymes, structural proteins, carrier proteins, protein feeds, receptor proteins.Consequently, the DNA must be some way to encode the structure of proteins and direct the order of the amino acids in them.
Amino acids joined to each other by a peptide bond which is formed by the condensation of amino groups (NH2) of one amino acid with a carboxyl group (COOH) with another amino acid.The amino acid sequence in the polypeptide chain of amino acids is written with a free NH2 group to amino acid with a free COOH-group.
Scientists have found that the code is a triplet, this means that each amino acid is encoded by the nucleotide triplet.Indeed, as the proteins used to construct the 20 different amino acids, the code can not be single nucleotide, since there are only four nucleotides.The code can also be dinucleotides, since only 16 possible combinations of 2 nucleotides.At 3 nucleotides the number of combinations increases to 64, and that is enough to encode the 20 different amino acids.Furthermore, it also follows that the genetic code is to be degenerate, namely one amino acid may be encoded by more than one triplet of nucleotides.Another important feature of the genetic code is that it is non-overlapping, with each successively new amino acid polypeptide chain encodes sequentially a new DNA triplet.The genetic code does not contain punctuation marks, and encoding triplets follow each other.The genetic code is universal and is used the same way as the prokaryotes and eukaryotes.The coding nucleotide triplets are called codons.
most important first two nucleotides of each codon.The third nucleotide nonspecific.Three codons define the signal termination of polypeptide chain synthesis (translation termination): UAA, UAG and CAA.This means that at the location of the messenger RNA (mRNA), where any of these codons polypeptide chain synthesis is terminated.Codons indicating the termination of synthesis of polypeptide chains, called stop codons.
Messenger RNA and process of transcription
should explain why it was so necessary to introduce the concept of messenger RNA.As is known, the DNA contained in the chromosomes of the cell and therefore in the nucleus, and the protein is synthesized in the cytoplasm of cells.In order that information on the protein structure recorded in the language of DNA fell into the cytoplasm, it is first overwritten (transcribed) to the mRNA molecule.
RNA differs from DNA in that chain RNA ribose sugar residue represented by (hence its name), thymine is replaced by uracil which has approximately the same complementarity to adenine as thymine.
To cheating occurred nucleotide sequence of the gene encoding the primary structure of the protein polypeptide chain at specific mRNA to a DNA chain at a distance from the gene to a specific sequence of nucleotides, called the promoter, joins special enzyme - RNA polymerase.
The starting point is the transcription of DNA base corresponding to the RNA base, which is the first to be included in the transcript.Transcription mRNA lasts as long as RNA polymerase II does not meet the termination signal (end) transcription.
biosynthesis polypeptide chain
The polypeptide chain occurring decoding information encoded by the genetic code, and build on the template mRNA of the polypeptide chain of a protein.This process involves two more types of RNA - ribosomal (rRNA) and transport (tRNA).For both types of RNA are numerous in the genome of genes on which the matrix synthesized RNA.
Education polypeptide chain of successively delivered to the mRNA tRNA with the corresponding amino acids occurs in the ribosomes.
gene structure in higher organisms is quite complicated.It comprises a promoter comprising a transcription initiation site, exons and introns.Contain exons encoding gene sequence, introns function remains unknown.At the boundary of exons and introns located consensus sequence that is recognized by splicing enzymes, ie. E. Enzymes for excision of introns from the primary mRNA transcript.At the 3 'end of the gene in the noncoding portion located site providing the addition of adenine residues 100-200 mRNA to ensure its stability.For the gene characterized by the so-called open reading frame, t. E. The existence of a sufficiently long sequence of triplets coding for amino acids, do not interrupt the stop codons or nonsense triplets.